In known communication systems, a modem is used at a terminal to connect the terminal to a public land mobile network (PLMN) such as a mobile cellular network (i.e. a 3GPP network or other CDMA network). The terminal may comprise a “host” or “application” processor that controls or performs processing functions on or for messages received from the network for the terminal.
To assist the application processor in receiving messages from the PLMN, the modem can receive a message addressed to the application processor from the PLMN. The message may be a short message service (SMS) message. The first part of reception is done by the modem. Following reception of the message, the modem indicates the content of the received message to the application processor. The application processor then reacts to the indication (such as by storing the SMS or indicating it to a user of the terminal). The application processor sends a response to this indication back to the modem. The response provides information on whether or not the SMS has been correctly processed by the application processor. If the SMS was correctly processed, the modem indicates such to the network using an acknowledgement. When the network receives such an acknowledgment from the modem, the network will not retransmit the message.
The above described process can be implemented using protocols provided by the 3GPP specifications. In particular, the modem may indicate the content of an SMS message to the application processor using an attention (AT) command port by asserting “+CMT: . . . ” (see TS27.005—chapter 3.4.1). After the modem has received this indication and reacted to it, the modem can respond to the modem by indicating that the SMS was correctly processed by asserting “AT+CNMA” on the AT command port (as described in 3GPP TS27.005—chapter 3.4.4). On reception of the AT+CNMA message, the modem confirms receipt of the received message to the network.
In parallel to indicating the message to the application processor (using, e.g. the +CMT indication), the modem starts a timer. The timer has a time interval ti associated with it. The time interval ti is a maximum time in which the modem expects to receive a response from the application processor to the indication. If a response is not received in time interval ti the modem is configured to respond to the network by indicating a negative result to the received message. The network may then re-transmit the message. Whether or not the network re-transmits the message is determined on a plurality of different factors. For example, the negative result may instruct the network to not transmit any further messages to the application processor.
Once the time interval ti has expired (without receipt of response from the application processor to the indication) the modem is arranged to stop indicating to the application processor that a message has been received e.g. the +CMT indication on the AT-port stops being asserted. If the response to the indication from the application processor arrives at the modem after expiry of the time interval ti the modem will handle that response as though the response is an erroneous transmission. For example, in 3GPP, on receipt of the AT+CNMA command being received at the modem from the application processor, the modem responds to the AT+CNMA command by responding ERROR to the application processor. This process is described in 3GPP TS27.005—chapter 3.4.4.